After a lot of tweaking, the problems with the cart, checkout, and other things seem to be fixed. I was able to put an item in the cart, view the cart, and go to the checkout. Everything appeared normal, including the reward points and payment functions. Please try to check out normally, but let me know (e-mail firstname.lastname@example.org) , if there are any problems with the cart or anything else. I really hope this has fixed the problems!
Thank you for your patience and understanding.